1. Field of the Invention
This invention relates to electrically conductive covers made by combining two or more of cases used in optical equipment such as cameras, and in household electric appliances, computers, word processors or measuring equipment.
2. Related Background Art
Traditionally, optical equipment such as cameras, and in household electric appliances, electronic equipment such as computers and word processors or measuring equipment are required to have outer covers which have shielding properties against electromagnetic wave which is generated from the inside of electric equipment such as high frequency oscillation circuit, motors or Brown-tube so that the electronic wave will not leak outside the equipment. In particular, with the recent tendency of electronic equipment to become smaller and lighter, the outer covers are made of plastics, making it an important subject to take shielding measures against electronic wave. Conventionally, the electromagnetic shielding of plastic covers has been made by such measures as spray coating using electroconductive paints, zinc spraying, electroless copper plating, Al vacuum vapor deposition and the use of conductive plastics.
Incidentally, the outer cover of such electronic equipment is usually formed by combining more than one case. To provide the entire outer cover with electromagnetic shielding properties, it is necessary to have electrical continuity through each case. For example, if a case with electroless copper plating and nickel plating is to be used as shown in FIG. 4A, the mere combining of the cases with plated surfaces coming into contact each other gives insufficient adhesion. Since electrical continuity cannot be provided in this event, electromagnetic wave leaks out of the connecting part. Therefore, using the lead wire 43 and the washer 44 as a conventional means to achieve electrical continuity through two or more cases in the cover, for example, the cases have been combined. Namely, the two cases were connected electrically by using lead wire and washer to strengthen the continuity which the mere butt joint of the two cases do not achieve sufficiently. However, this method is getting hard to apply since the securing of space for lead wire arrangement and for the fixing of the washer is becoming difficult with the recent tendency to made products smaller. Furthermore, there is a problem that the increase in the number of parts results in higher costs. Another method to have electrical continuity through the cases is to improve the adhesion of the connecting part by applying silver paste and the like onto the connecting part of the cases without using the lead wire and washer. This method causes a cost problem since it requires the coating of silver paste to the connecting part in its entirety leading to a substantial increase of work steps. Besides, the plastic cases to which electroless copper plating is applied require decorative coating prior to the assembling of the cover. With respect to the connecting part of the case and fixing part of the washer, masking with tapes and fixtures is necessary to prevent the adhesion of coating layer at the time of coating. As a result, the number of work steps increases substantially and costs become higher so that mass-producibility is substantially precluded.
Plastic base materials electrolessly plated with copper have usually used electroless nickel plating on the copper plating to prevent corrosion of the copper. The surface of this nickel plating does not adhere sufficiently to the decorative coating layer. For satisfactory adhesion, the use of special paints (for example, ORIGI PLATE of Origin Electric K.K.) is necessary. This causes a significant problem of costs.
Another method is to provide electromagnetic wave shielding along with decorative coating by giving spray coating of conductive paints to a plastic case. In this case, the layer thickness of at least 50 .mu.m is necessary to secure the electromagnetic wave shielding, with the result that there remains a problem of poor accuracy at the connecting part of the cases. Further, the coating on the connecting part of the cases is liable to peel off, making the continuity between the cases insufficient.
If conductive plastic is used, there is no conductivity at the connecting part since the conductive filler does not come out onto the surface. To gain electrical connection between the two cases, a known method is to connect by the use of a case damping screw as shown in FIG. 4B. The conductive plastic in which conductive filler is mixed into resin is used, along with the use of the damping screw, to connect the two cases and at the same time to provide electrical connection between the cases by improving the contacting properties of the conductive filler. This method, however, requires the processing of the cases beforehand for the use of the damping screw, causing such problems as an increase in the number of work steps, weight increase of conductive covers and increased cost. Also, the cost of conductive plastic materials themselves is high and the mass-producibility is poor. Thus, in response to the recent, growing tendency of electronic equipment becoming smaller and lighter, there is a demand for such conductive cover to be light in weight, to require small space, to be produced in a smaller number of work steps, to have excellent electromagnetic shielding properties and to have a decorative coating layer excellent in coating properties.